Dominic Stone
Cardiac muscle, also known as myocardium, is one of the three types of muscle tissues in the human body, the other two being skeletal and smooth muscle. Cardiac muscle is made up of sarcomeres that allow for contractility. Sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin. The myofilaments slide past each other as the muscle contracts and relaxes, forming a cross-bridge and causing the contraction of the heart.
| Characteristics | Values |
|---|---|
| Cardiac muscle cell shape | Tubular structure |
| Cardiac muscle cell size | 100 μm long and 10–25 μm in diameter |
| Cardiac muscle cell composition | Chains of myofibrils |
| Myofibril composition | Repeating sections of sarcomeres |
| Sarcomere composition | Thick and thin filaments (myofilaments) |
| Thick myofilament composition | Myosin protein |
| Thin myofilament composition | Actin protein |
| Contraction mechanism | Sliding filament theory |
| Contraction trigger | Release of calcium from the sarcoplasmic reticulum |
| Calcium function | Binds to troponin, allowing actin-myosin interaction |
| Calcium removal | SERCA pump sequesters calcium into the sarcoplasmic reticulum |
| Cardiac muscle type | Striated muscle |
| Cardiac muscle control | Involuntary |
| Cardiac muscle layers | Three (pericardium, myocardium, endocardium) |
| Cardiac muscle cell junctions | Intercalated discs |
| Intercalated disc components | Gap junctions, desmosomes, fascia adherens junctions |
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What You'll Learn
Cardiac muscle is made up of sarcomeres
Cardiac muscle, or myocardium, is one of three types of muscle in the human body, the other two being smooth muscle and skeletal muscle. The myocardium makes up the thick middle layer of the heart, with the other two layers being the thin outer layer called the epicardium (or visceral pericardium) and the inner endocardium. The cardiac muscle is responsible for the contractility of the heart, which allows it to pump blood through the body.
Cardiac muscle cells, or cardiomyocytes, are tubular structures composed of chains of myofibrils, which are rod-like units within the cell. Myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of thick and thin filaments called myofilaments, which are made up of the proteins myosin and actin, respectively. The thick myosin filaments are the molecular motors that pull on the long actin filaments to produce muscle contraction. The sliding of actin and myosin past each other produces the formation of "cross-bridges," resulting in the contraction of the heart and the generation of force.
The length of sarcomeres is important in regulating the force of contraction of the heart. As the sarcomeres within the myofibrils are stretched, there is an increase in the force of contraction, or tension development by the muscle fiber. This is due to the increased overlap between actin and myosin, which creates more sites for the hydrolysis of ATP by the myosin ATPase. The chemical and physical interactions between actin and myosin cause the sarcomere length to shorten, leading to the contraction of the muscle fiber.
The arrangement of actin and myosin filaments in cardiac muscle is similar to that of skeletal muscle, giving both types of muscle their striated appearance. However, unlike skeletal muscle, cardiac muscle is under involuntary control, with its own auto-rhythmicity due to the presence of pacemaker cells. These pacemaker cells, such as the sinoatrial (SA) node, spontaneously depolarize to initiate the contraction process.
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Sarcomeres consist of thick and thin filaments
Cardiac muscle, also known as myocardium, is one of three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. The heart is made up of three layers, with the myocardium forming the thick middle layer. The myocardium is surrounded by a thin outer layer called the epicardium (or visceral pericardium) and an inner endocardium.
Cardiac muscle is a type of striated muscle, characterised by a similar arrangement of actin and myosin filaments to mediate contraction. Cardiac muscle, like skeletal muscle, is made up of sarcomeres that allow for contractility. The sarcomere is the fundamental repeat unit within muscle that is responsible for contraction. The sarcomere consists of thick and thin filaments, called myofilaments. The thick myofilaments contain the protein myosin, while the thin myofilaments contain the protein actin.
The thin actin filaments lie between the two thick myosin filaments. The outside of the cardiomyocyte is surrounded by a plasma membrane called the sarcolemma, which acts as a barrier between extracellular and intracellular contents. The sarcolemma contains voltage-gated calcium channels, which are specialised ion channels that skeletal muscle does not possess. The released calcium attaches to troponin C, causing tropomyosin to detach from the myosin-binding sites on actin. Actin and myosin then form a cross-bridge, and contraction occurs.
The sliding filament model proposed by Huxley and Hanson in 1954 explains how sarcomeres and thus muscle could be shortened by sliding thick and thin filaments past one another. This process is ATP-dependent, requiring direct interaction between the myosin head and actin. The sliding of actin and myosin past each other produces the formation of "cross-bridges", which causes contraction of the heart and the generation of force.
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Thin filaments contain the protein actin
Cardiac muscle, or myocardium, is one of three types of muscle in the human body, the other two being skeletal and smooth muscle. It makes up the thick middle layer of the heart and is responsible for the contractility of the heart and, therefore, the pumping action. Cardiac muscle cells are tubular structures composed of chains of myofibrils, which are rod-like units within the cell. The myofibrils consist of repeating sections of sarcomeres, which are the fundamental contractile units of the muscle cells.
Sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin. The thin filaments are made up of two helically arranged filamentous polymers of the protein actin, with a long filamentous protein tropomyosin that lies in the grooves of the helix. They also contain an associated globular protein, troponin, found at intervals along the filament. Each globular actin monomer contains a binding site for the globular myosin head.
The thin filament contains several important contractile regulatory proteins, with the main component being the actin filament, which is formed from the polymerization of globular actin molecules. Actin has the ability to polymerize to form long filaments, converting monomeric G-actin to polymeric F-actin. Actin also has the ability to bind myosin and activate its MgATPase activity. The sliding of actin and myosin past each other produces the formation of "cross-bridges", which causes contraction of the heart and generation of force.
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Actin and myosin form cross-bridges during contraction
Cardiac muscle, also known as myocardium, is one of the three major categories of muscles in the human body, the other two being smooth muscle and skeletal muscle. Cardiac muscle is made up of sarcomeres, which are the fundamental contractile units of the muscle cells. Sarcomeres are composed of long proteins that organize into thick and thin filaments, called myofilaments. Thin myofilaments contain the protein actin, and thick myofilaments contain the protein myosin.
To enable a muscle contraction, tropomyosin must change conformation, uncovering the myosin-binding site on an actin molecule and allowing cross-bridge formation. This can only happen in the presence of calcium, which is kept at extremely low concentrations in the sarcoplasm. If present, calcium ions bind to troponin, causing conformational changes in troponin that allow tropomyosin to move away from the myosin-binding sites on actin. Once the tropomyosin is removed, a cross-bridge can form between actin and myosin, triggering contraction. Cross-bridge cycling continues until Ca2+ ions and ATP are no longer available and tropomyosin again covers the binding sites on actin.
The sliding of actin and myosin past each other produces the formation of "cross-bridges", which causes contraction of the heart and generation of force. The globular heads of myosin bind actin, forming cross-bridges between the thick and thin filaments. This movement slides the actin filaments from both sides of the sarcomere toward the M line, shortening the sarcomere and resulting in muscle contraction.
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Cardiac muscle is striated and involuntary
Cardiac muscle, also known as myocardium, is one of three types of muscle in the body, the other two being skeletal and smooth muscle. The myocardium forms the thick middle layer of the heart and is responsible for the heart's contractility and pumping action.
Cardiac muscle is striated, meaning that it appears striped. This is due to the arrangement of actin and myosin filaments within the sarcomeres of the muscle cells. The sarcomeres are the contractile units of the muscle cells, composed of long proteins that organise into thick and thin filaments called myofilaments. The thick myofilaments contain the protein myosin, while the thin myofilaments contain actin. The interaction of these filaments forms the basis of the sliding filament theory, which explains how muscle contraction occurs.
The striated appearance of cardiac muscle is similar to that of skeletal muscle. However, unlike skeletal muscle, cardiac muscle fibres connect at branching, irregular angles called intercalated discs. Cardiac muscle is also under involuntary control, meaning that it contracts and relaxes without conscious direction. This is in contrast to skeletal muscle, which is under voluntary control.
The contractile function of cardiac muscle is vital for pumping blood throughout the cardiovascular system. The contraction of cardiac muscle occurs through the excitation-contraction coupling (ECC) process, which involves the release of calcium from the sarcoplasmic reticulum. This calcium attaches to troponin C, causing the actin and myosin filaments to form cross-bridges and resulting in muscle contraction.
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Frequently asked questions
Yes, cardiac muscle has actin. Actin is a protein that forms thin filaments, which slide past thick filaments (made of myosin) during muscle contraction and relaxation.
Actin, along with myosin, forms the basis of the sliding filament theory, which explains how cardiac muscle contracts. During contraction, actin and myosin form cross-bridges, with the help of calcium ions, leading to the generation of force and muscle contraction.
Cardiac muscle, also known as myocardium, is one of three types of muscle tissues in the body, including skeletal and smooth muscle. It forms the thick middle layer of the heart, surrounded by the outer pericardium and inner endocardium layers. Cardiac muscle is composed of individual muscle cells, called cardiomyocytes, which are highly coordinated and contract involuntarily to pump blood throughout the body.
While both cardiac and skeletal muscles are striated and use actin and myosin filaments for contraction, they have some key differences. Cardiac muscle is under involuntary control, while skeletal muscle requires neural input for contraction. Additionally, cardiac muscle cells are linked into long chains by specialized cell junctions, unlike skeletal muscle cells, which fuse to form syncytia.
The contraction of cardiac muscle is regulated by the concentration of calcium ions (Ca2+). Calcium is released from the sarcoplasmic reticulum during electrical stimulation, binding to troponin and tropomyosin. This interaction uncovers binding sites on actin, allowing myosin to bind and pull the filaments, resulting in muscle contraction. When calcium concentration falls, the binding sites on actin are covered again, causing the muscle to relax.
